Add MERRA-2 Layers

[minniewong]

Description

Fixes nasa-gibs/worldview#1122.

Added 25 MERRA-2 layers

Checklist

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• I have read the CONTRIBUTING doc
• I have added necessary documentation (if appropriate)
• I have added tests that prove my fix is effective or that my feature works
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Further comments

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@nasa-gibs/worldview

[mike-mcgann]

👍